Presenting Pump Power Peculiarities, Playing With Pumps And Pipes

Pump power exhibit at the Damen Dredging Experience
Pump power exhibit at the Damen Dredging Experience

Hej kära läsare, jag vill ta dig till ett land långt borta, för länge sedan. Min älskade Sverige.

In 1996, I started my graduation with Skanska1 in Sweden. They had a project to clean up a lake2 with an auger dredge. The auger was not performing and they asked the Delft University of Technology to investigate the problem and write a report with my solution. Off, I went to Växjö and spent a year on a dredge. During my reconnaissance of the project in the first week, I noticed that the flow in the pipe line was very slow and the motor was hardly working at full speed. As an innocent student, I asked where they were pumping the material to. ‘Oh, through 7 kilometre of pipe and 30 meter up into the hills.’ They were lucky it was such a fluid material and did not settle at such a low velocity. I then proposed they should buy a booster station to increase production3, as I could not see anything wrong with the auger. ‘No, no. It has to be the auger and the engine is strong enough; you see, there is no power required.’

Clean up project at Södra Bergundasjön near Växjö
Clean up project at Södra Bergundasjön near Växjö

That was the first time I saw the slow flow fallacy at work. Intuitively you would expect, that a long pipeline would require more power to transport the mixture than a short pipe line. This is exactly what this exhibit is trying to visualise. Water can be pumped through either the short or long pipe. From the lines on the tank wall, you can read that the output velocity of the fluid is about 1.5 m/s. In the vertical pipe, the delivery pressure is indicated. Multiplying pipe velocity and fluid pressure results in the actual power in the pipe line. The pump has to provide this power, by converting electrical power to mechanical power and eventually fluid power. On the display at the left of the buttons, the consumed electrical power can be read.

Discharge capacity through the short pipe line
Discharge capacity through the short pipe line

When you select the short pipe line, you have to notice the higher flow velocity and the required power at the display. Switching over to the longer pipeline, you will notice a drop in velocity. Due to the longer pipe, the fluid experiences more resistance. For the same pressure, the flow will be lower. Consequently, the power consumption will be lower also! This is exactly according to the theory. A pump at a lower capacity will consume less power, even if the pressure rises slightly.

Discharge capacity through the long pipe line
Discharge capacity through the long pipe line

Off course, the delivered mixture will be less, on the longer line. You might increase the speed of the pump to have more pressure. And indeed, that would require more power. But there is a maximum speed on the pump drive. Same for a very short pipe. You might end up below the idle speed of the diesel engine. Be careful in your project layout that you do take into account this viable operating range for the length of the pipe line. A longer pipeline might require a booster station for increased production. Conversely, a shorter pipe line might be chosen with a smaller diameter for increased resistance and lower power consumption, while keeping the operating point of the dredge pump near the Best Efficiency Point.

Graphical explanation of the power consumption for identical pump speeds
Graphical explanation of the power consumption for identical pump speeds

And the Swedish dredge on 7 km of pipeline? It turned out, it was not a technical problem. They had no hurry. The contractor was hired per week on an open contract…

Auger dredge 'Detritus'
Auger dredge ‘Detritus’

References

  1. Welcome to Skanska, Skanska
  2. Södra Bergundasjön, Wikipedia (Swedish)
  3. Damen booster station, Damen

See also

A Virtual Tour Through The Dredging Experience In Damen Nieuws

Head line of the virtual tour through the Damen Dredging Experience in Damen Nieuws

One of the things I like about my work is to tell people about it and explain all the delicate details of dredging in a fashion that everybody should be able to comprehend the complexity of moving sand. I am very fortunate to work at Damen as they gave me my own museum to care for: The Damen Dredging Experience1. I’ve discussed several of the exhibits in the Experience on this website already2. Since our last appearance in the internal Damen Nieuws magazine3, the intriguing science of dredging caught the attention of the editorial board. And what better way to share our knowledge on dredging than reporting about it in our Damen Nieuws.

Here on my own website, I already introduced ‘the Experience’ about two years ago, intending to cover the various exhibits regularly. Alas, reality not always behaves as planned. There were so many other interesting items to share, or current CEDA events, or urgent requests from the service department. Well, this publication is a good incentive to continue my coverage of the Damen Dredging Experience.

First exhibit in the entrance hall: a real size DOP pump
First exhibit in the entrance hall: a real size DOP pump (Credit: Judith Korver)

One part of the experience, that does not lend itself for a very detailed explanation is the entrance to the building and the journey to the fourth floor where the experience is located. Immediately after entering the building, you may be overwhelmed by the DOP pump suspended from the ceiling. DOP pumps by themselves are not very large. But, when they are displayed out of place like inside a building, they are still formidable machines. Visitors can be introduced to the general composition of our dredging equipment with this DOP pump: it is basically a small scale dredging machine. It has a suction mouth, a dredge pump and a construction to keep everything together. Same follows for a CSD or even a hopper dredge.

Pictures about the history of The Netherlands and dredging used in the wall illustrations
Pictures about the history of The Netherlands and dredging used in the wall illustrations

Next are four floors with murals depicting topics from the dredging industry. First floor is covering the history of dredging in the Netherlands. How we always had to battle the sea to keep our land dry and tame the rivers for infrastructure. This long history made The Netherlands an excellent breeding ground for knowledge on dredging. The blooming industry is backed up by centers of excellence as the TU Delft, Marin and Deltares. With each of which we as Damen have cooperated to develop our products.

Pictures about the historical relation between Damen and dredging on the second floor
Pictures about the historical relation between Damen and dredging on the second floor

Within the Dutch dredging industry Kommer Damen played a pivotal role in the supply of auxiliary equipment to the big dredging contractors. Eventually, the company De Groot Nijkerk was acquired to also deliver dredging equipment directly.

Pictures with Damen products used in projects around the world
Pictures with Damen products used in projects around the world

Our products have been applied world wide on small and large projects. Everywhere where some wet infrastructure was developed, Damen has supplied some or more equipment. It always makes me proud to see the result of a big project and know that in some (remote indirect) way I have contributed to it.

Pictures illustrating the future of dredging industry
Pictures illustrating the future of dredging industry

The last mural is about the future of the dredging industry. One example is the increasing demand of raw materials. Beginning with the most basic of all: sand for the concrete industry. Increasingly, we see more demand for more exotic materials that are even harder to get: sea floor minerals from the abyss of the ocean. The other example is the ever increasing environmental awareness of society and the attention of cleaning up contamination and creating value for nature. Each of which our DOP pumps form the beginning is excellently purposed.

The Damen Dredging Experience is available for anyone who wants to visit, although it is best to request an appointment with one of our staff4.

Overview of the exhibits in the ‘Damen Dredging Experience’
Overview of the exhibits in the ‘Damen Dredging Experience’

References

  1. Damen Dredging Equipment in Nijkerk Officially Open, DredgingToday
  2. Dredging Experience, Discover Dredging
  3. Our Interview About New Pump Designs In The Latest Damen Nieuws
  4. Contact, Damen Dredging Equipment

See also

Experiencing The Cutting Edge Of Dredging Technology

Exhibit on cutting forces at the Damen Dredging Experience
Exhibit on cutting forces at the Damen Dredging Experience

Ever tried to eat peas with a knife? Didn’t fare well, right? Sorry, this post will not help you. It is just more entertaining doing so. And you might remember this week’s lesson.

After all the other posts, I would like to continue our tour along the exhibits of the Damen Dredging Experience. We’ve seen at “the Bank” that by gravity and hydraulic action the sand can start moving towards the suction mouth. Another well-known mechanism for collecting the sand is by cutting. This mechanism is primarily used in the cutter heads of CSD’s of course, but also in the trailing drag heads of TSHD’s.

Be aware, that the gravel sample in the exhibit is an artificial sediment, specifically designed to be porous and demonstrate the “rolling peas effect”. Naturally occurring gravel sediments have a wide range Particle Size Distribution and will have virtually no gaps between the grains. The smaller grains will lock the bigger grains in place and it will be more difficult to move them.

In this exhibit you will see two different types of sediment. Wet fine grained sand and very coarse gravel in a jar with a handle. When you rotate the blade in the gravel, you will notice a rather high cutting force, that remains relatively constant. The water can easily move through the pores. Rotating the blade in the jar with sand, is very hard at first. But as soon, as there is a chunk of sand dislodged, the cutting force is decreased dramatically. The decrease can be explained by the fact, that once water has entered the shear plane, it cannot dissipate back in the pores and will lubricate the chunk moving over the bed. This is a phenomenon, sometimes encountered with ploughs.

Under pressure in densely packed sediments
Under pressure in densely packed sediments

The theory of cutting sand is largely explored by dr. S.A. Miedema1. He wrote an extensive book2 on this subject and anyone interested in the details is encouraged to read it. Again, the basis for cutting sand is the dilatancy of sand, just like in the previous exhibit. The grains are moved and the water has to be forced in the pores. As the blade moves at a certain speed, the sand exerts a horizontal cutting force on the blade. Force, times speed is power. At this speed and cutting height you have a certain production. Power divided by production yields a Specific Cutting Energy3, which is a parameter for how much effort it costs to cut 1m³. The SCE is largely governed by the undrained shear strength and the angle of internal friction and is different for every type of sediment. They are measured with a Cone Penetration Test4,5. In order to estimate the production of the dredge, we really need to know these parameters. If they are not available, maybe you can receive the results from a Standard Penetration Test6,7.

Basic explanation of the theory of sand cutting
Basic explanation of the theory of sand cutting

From the equations, you can derive that for a hard material, the SCE can be quite high. Consequently, with a known installed cutter power, the production Q will result quite low. From this perspective, there is no upper limit in the hardness of the soil, anything can be cut. It is just, that the resulting production might be too low for a viable business case. In this respect, it is always difficult to say the maximum hardness of the soil the CSD can cut. Usually, the increased vibrations and unsteady process will limit the productivity in such circumstances.

Example of a cutter production for a CSD
Example of a cutter production for a CSD

This provokes a nice practical experiment for you at home or when you have you have to entertain guests at dinner: have a nice recipe with big peas and fine grained rice8. Serve the peas and rice separately and notice the variation of effort to stir the ingredients separately. Then, mix a portion together and notice the increase in cutting force. For enhanced realism, add some sauce. Exclaim your amazement to your perplexed table partners and explain that you are not playing with your food, but are on a study assignment for your work. Bon Appetit!

Ingredients for a pea and rice recipe
Ingredients for a pea and rice recipe

References

  1. dr. S.A. Miedema, TU Delft
  2. The Delft Sand, Clay & Rock Cutting Model, Dredging Engineering
  3. CEDA Webinar Specific Cutting Energy, CEDA
  4. Cone penetration test, Wikipedia
  5. Painted Hills, how to unveil the sediment layers below the surface, Discover Dredging
  6. Standard penetration test, Wikipedia
  7. Lessons in Camping: Basic Soil Investigation, Discover Dredging
  8. Nice rice-a-pea, Albert Heijn

See also